The impacts of nanotechnology on the improvement of liquid insulation of transformers: Emerging trends and challenges

Highlights

• The use of nanotechnology and nanoscience in transformer insulation was manifested in details.

• The transformer oil-based nanofluids advancement in details

• Summarizing past research, identifying research types, foci and anticipating some future research directions

Abstract

The rise in transmission voltages levels, the demand for insulating reliability of the transformer is getting more and more critical. The mineral oil (MO) together with paper is the main insulation constituents in oil-immersed transformers. To enhance the insulation level of ultra-high voltage (UHV) transformers and to decrease its weight and size, it is highly required to increase the insulating conduct of transformer oil (TO) and oil-impregnated cellulose. Lately, a distinctive novel venture of application of nanotechnology with liquid insulation of transformer has been conducted and the results have displayed marvelous enhancement in the insulation properties of transformer oil. The transformer oil-based nanofluids research domain (i.e., NFs) has been attracting huge attention during recent years both in theoretical and practical fields. NFs may be applied as a potential substitute for MO in the near future. To recognize various brunts of NFs and show evolving developments and challenges, the study demonstrates a scientometric review established on 210 published features from 2004 to 2019 through co-author, co-occurring and co-citation investigations. The significance of the editorial lies in compiling former inquiries, categorizing research nature, motivations and forecasting impending research orientations. Lastly, some recommendations for future NFs research and application are suggested.

Introduction

Electricity is considered the basic need of human society and it is generally the backbone for economic growth and prosperity of society. The smart grid is the one that integrates metropolises and entertains social accomplishments pairing with economic, communal and ecological systems with population growth and sprawl. Furthermore, the electric system grants towards socio-economic growth and enriched standard of living through generating inter- to intra-city networks during suburbanization. Therefore the evolution of prospectus high voltage networks has raised demands on conduct and reliability of insulating constituents applied in electric systems to survive with vigorous and elusive working circumstances [1]. The transformer which transfers energy and transforms voltage levels is the most indispensable element of an electric network. The failure of this essential component may be disastrous [2]. Many of transformers in operation around the planet are impending their projected life; consequently, it is in the prime focus of researchers to enhance the operational security of these entities [3]. All possible transformer failure facts confirm that lifespan of the transformer is highly dependent on its insulation system and the life of units which were failed due to insulation issues is 17.8 years, which is practically half of anticipated life expectancy of 35 to 40 years [3,4]. The life and reliability of transformers generally depend on stature and attributes of insulation material [[5], [6], [7], [8]]. Fig. 1 indicates the data for the key sources of transformer breakdowns based on 964 prominent failures. According to transformer reliability survey, 2015 by Council on Large Electric Systems (CIGRE), it was concluded that the major reason for transformer collapse was insulation failure.

MO/paper insulation system has been broadly applied as the major insulating material in HV equipment, such as cables, transformers, capacitors, circuit breakers, bushing etc. due to their high mechanical strength and better dielectric traits [9] (Fig. 2). However, the demand for a smaller size, larger capacity and high reliability of equipment by the electric industry has challenged the electrical attributes of this typical insulation system. In MO/paper insulation system, distinction between permittivity of liquid (~2.2) and solid one (ranging, 3.6–4.5) has remained a crucial matter that might restrain the compacted design of HV apparatus since the liquid insulation endure a greater strain than the solid insulation at alternating current (AC) and/or lightning impulse (LI) voltages, however breakdown strength (BDS) of the liquid insulation is generally lower than that of solid one.

The liquid insulation typically performs twofold general functions in HV machinery, i.e., cooling and insulation. MO has been applied as an insulating and cooling medium for above a century in HV machinery owed to its thermal and insulating quirk [10]. Thermal and insulating traits of MO generally restrict size miniaturization and maximal power transfer [10,11]. However, refined MOs commonly used as insulating fluids in the HV apparatus have lower thermal conductivity and hence, conclude lower cooling performance [12].

Transformer oil (TO/MO) is one of the most critical components of transformer, which accomplished the prior mentioned two fundamental objectives: as cooling liquid, it stands useful in transmitting heat originated in vigorous elements (magnetic core and windings) towards the outer walls of transformer where it may possibly be depleted; as an insulating medium, MO obstructs course of current outside of electrical components [13]. A small value of thermal conductivity of MO induces constraints in the operation of transformers, because during loading conditions, the huge rise in temperature may lead to an extreme local upsurge in temperature in MO (hotspot), hence performance of MO is restrained.

Through brisk expansion of the electric power structure, transmission capacity and voltage level are increasing which not simply increases weight and volume of power machinery constantly nevertheless condenses its reliability and security. The oil-cellulose insulation system as an established insulation tool has existed generally in electrical apparatus but it is exposed to serious integrated aging issues, particularly thermal aging problems due to severe thermal, electrical and mechanical stresses [14]. Generally, MO has been applied as insulating and cooling material for nearly above a century in transformers due to its thermal and insulating attributes [15]. However, MO has the tendency to catch flame or even blast accidents in power apparatus because of polycyclic aromatic hydrocarbons present in it, which don't conform to essentials of fire and security. MO is normally a derivative of petroleum yields which are non-sustainable source and it is considered to be harmful to atmosphere. Vegetable oil (VO), which is an ecofriendly fluid dielectric, has captured massive recognition as a prospective substitute to conventional MO for electric equipment [16]. Conversely, to cope with growing demand for HV rate and lesser dimensions for electrical apparatus, progress of TO with encouraging dielectric and thermal traits is mandatory [[17], [18], [19]].

Nanofluid (NF), a very familiar term these days, has been in the prime focus of the research community in the past couple of decades. This term was first coined by Choi et al. in 1995 [20]. It is generally defined as a fluid with nano-sized particles suspended homogeneously at a few weight percentages (wt%). Nevertheless, in the HV liquid insulation research, the term nano-liquid and nanofluid are used interchangeably to refer to MO/nanoparticle (NP) mixture for cooling and insulation interest. NFs, a revolutionary liquid with better thermal conductivity paralleled to conventional liquids. The thermal conductivity enhancement of NFs brands them suitable applicants for performance improvement of thermal equipment, even though simultaneously the rise in viscosity (which occurs due to the addition of NPs to base fluid) could be an undesirable feature of NFs application. NFs finds many application in solar energy devices [21,22], micro-channel heat sinks [23,24], car radiators [25,26], medicine [27], different cavities [28,29], and so forth.

The nanotechnology application in HV engineering has been generally oriented towards the manufacturing and characterization of the so-called nano-dielectric materials [30]. Nanodielectric (ND) has attained huge attention due to the present development and progress in nanotechnology after 1990s [31,32]. The research indicated that the adding of NPs significantly enhances the insulation life expectancy of solid polymers. A similar technique may be used for dielectric liquid insulation with the aim to improve its thermal and insulating features. Nevertheless, in the beginning, traditional micron-sized particulates were suspended into liquids for the improvement of thermal conductivity [33]. The results showed a subsequent decline in BDS [34].